In preparing liquid crystal display (LCD) device, liquid crystal cell comprising liquid crystal, and polarizer are basically required, and suitable adhesive layer or pressure-sensitive adhesive layer may be used for binding them. In addition, for improving functions of LCD device, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancing film, and the like may be used, with additionally adhered to the polarizer.
As a main structure, LCD device generally includes a uniformly aligned liquid crystal layer; a polarizer with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisting of a transparent glass plate or a plastic sheet material containing a transparent electrode layer; a phase retardation plate; an additional functional film layer; and the like.
The polarizer may include an iodine compound or a dichroic polarizing material aligned in a certain direction, in this case, to protect these polarizing elements, multi-layers may be formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizer may additionally include a compensation film for wide view angle such as a phase retardation film or a liquid crystal type film in a shape having a unidirectional molecular alignment.
The aforementioned films are made of materials having different molecular structures and compositions, and thereby have different physical properties. In particular, under a high temperature or high temperature and high humidity condition, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient. As a result, if the polarizer is fixed by a pressure-sensitive adhesive, then stress is concentrated on a TAC layer by shrinkage or expansion of the polarizer under a high temperature or high temperature and high humidity condition, leading to birefringence and thus light leakage. In this case, overall, negative birefringence occurs due to the shrunk TAC layer.
The light leakage phenomenon can be suppressed by preventing generation of residual stress with regulation of a stress releasing characteristic of the pressure-sensitive adhesive fixed on the polarizer. This can be achieved when the used pressure-sensitive adhesive has a non-crosslinked structure.
Traditionally, a pressure-sensitive adhesive layer have been used in the form of a partially crosslinked viscoelastic material to have a good high-temperature cohesive strength, thereby maintaining endurance reliability and showing suitable pressure-sensitive adhesive properties.
However, such a pressure-sensitive adhesive layer used in the form of a partially crosslinked viscoelastic material has residual stress under given stress and high polymers in the crosslinked structure are aligned in a particular direction, resulting in birefringence. In this alignment, an acrylic pressure-sensitive adhesive shows negative birefringence.
With a recent tendency to increase the size of LCD panel, the size of polarizer is also increasing, whereby the shrinkage of the polarizer is also to be increased and the residual stress of a pressure-sensitive adhesive layer is also to be increased under a heat resistance or moisture resistance condition. It results in large negative birefringence and severe light leakage. To minimize the light leakage phenomenon, it is necessary to perform optical compensation which regulates negative birefringence of the pressure-sensitive adhesive resulting from the residual stress into positive birefringence.
To perform the optical compensation, it is important to cause the pressure-sensitive adhesive to show positive birefringence under the residual stress by using a compound having high optical anisotropy and superior compatibility with the pressure-sensitive adhesive.
As an approach to improve the light leakage phenomenon, Japanese Unexamined Patent Publication No. 1998-279907 discloses a method for improving the light leakage phenomenon by mixing a high molecular weight acrylic polymer and a low molecular weight acrylic polymer having a molecular weight of less than 30,000 to provide a stress releasing characteristic generated from a polarizer. In this method, however, the effect of improving the light leakage phenomenon is not satisfactory as the size of the polarizer increases, the cutting characteristic is degraded due to low modulus, and the endurance reliability is reduced during long-term use.
Korean Patent Publication No. 2003-0069461 discloses a technical idea in which 0.01 to 40 parts by weight of a low molecular weight material representing positive birefringence under residual stress is mixed into an acrylic pressure-sensitive adhesive layer to correct negative birefringence of the acrylic pressure-sensitive adhesive layer which represents under residual stress. However, the low molecular weight material used herein has some problem in its compatibility with the high molecular weight copolymer and thus is likely to undergo phase separation with the high molecular weight copolymer when being used by a large amount, and may also have some problem in light transmission or endurance.
As the above, a stress releasing function is generally provided to a pressure-sensitive adhesive by adding a plasticizer or a low molecular weight material to a high molecular weight copolymer or by regulating a crosslinking structure, so as to improve light leakage. However, it is difficult to completely suppress the light leakage phenomenon merely by stress release.
That is, since a partial-crosslinking structure has to be introduced to pressure-sensitive adhesive for a polarizer in order to maintain the endurance reliability of the pressure-sensitive adhesive, the residual stress of the pressure-sensitive adhesive, resulting from the crosslinking structure, cannot be entirely removed. As a result, a traditional acrylic pressure-sensitive adhesive layer has negative birefringence under the residual stress, which is a major factor that makes it difficult to improve the light leakage phenomenon, together with negative birefringence of a TAC layer of a shrunk polarizer.